Photo resist as opaque aperture mask on multispectral filter arrays

ABSTRACT

An apparatus (e.g., a multi-spectral optical filter array, an optical wafer, an optical component) has an aperture mask printed directly thereon, the aperture mask including a positive or negative photoresist. The apparatus includes a substrate having the aperture mask printed on at least one of a light entrance surface or a light exit surface of the substrate so as to provide an aperture over a portion of the substrate. The photoresist from which the aperture mask is formed is photo-definable or non-photo-definable, and is deposited/printed to form the aperture mask on the substrate.

This application claims the benefit of U.S. Provisional Application No.62/557,909 filed Sep. 13, 2017 and titled “PHOTO RESIST AS OPAQUEAPERTURE MASK ON MULTISPECTRAL FILTER ARRAYS”. U.S. ProvisionalApplication No. 62/557,909 filed Sep. 13, 2017 and titled “PHOTO RESISTAS OPAQUE APERTURE MASK ON MULTISPECTRAL FILTER ARRAYS” is herebyincorporated by reference in its entirety into the specification of thisapplication.

BACKGROUND

The present disclosure relates to the optical arts, optical filter arts,spectrographic arts, and related arts.

Optical filters with high spectral selectivity can be manufactured usinga stack of layers, with alternating layers of two (or more) constituentmaterials having different refractive index values. Such filters aresometimes called interference filters, and can be designed to provide adesigned pass-band, stop-band, high-pass, or low-pass output. Forpass-band filters, the width of the pass band can typically be made asnarrow as desired by using more layer periods in the stack, albeitpossibly with some transmission loss at the peak transmissionwavelength. A notch filter can be similarly designed by constructing thestack of layers to form a Bragg reflector blocking the stop band. Thelayer stack is deposited on a substrate that is optically transmissivefor the wavelength or wavelength range to be transmitted, and may forexample be a glass plate for an optical filter operating in the visiblespectrum. This results in a filter plate whose structural rigidity isprovided by the substrate.

In such optical filters, a given filter plate operates at a single welldefined pass band or stop band. The layers of the stack are typicallyrequired to have precise thicknesses to meet the specified wavelengthand bandwidth for the pass band or stop band.

However, it is difficult or impossible to vary the layer thicknessesacross the substrate plate during layer deposition or by post depositionprocessing in a controlled manner in order to provide different passbands or stop bands in different areas of the plate. Such an arrangementis useful for a spectrometer, spectrum analyzer, or other “multispectral” applications.

Filter arrays address this problem by fabricating a set of filter plateswith different filter characteristics (e.g. different pass band or stopband wavelength and/or bandwidth). The filter plates are then diced toform filter elements in the form of strips. These strips are then bondedtogether in a desired pattern to form the filter array. The resultingfilter array is sometimes referred to as a “butcher block” due to itssimilarity in bonding structural elements (filter elements here, c.f.wood elements in the case of an actual butcher block). This approachdecouples the optical characteristics of each filter element of thefilter array from those of the other filter elements, enablingsubstantially any combination of filter elements in a single filterarray.

In the optical arts, if an image to be viewed is too bright, stray lightcan cause areas of bright light to bleed into adjoining dark areas andblur or eliminate the border between the bright and dark areas. This canbe particularly problematic when two bright areas are separated by anarrow dark area, because the irradiation can effectively cause theseparating dark area to disappear. This can result in too much glareexisting for any significant detail to be seen.

To alleviate the aforementioned problems, aperture masks are often used.A mask, which is often a dark color, is placed over the filter array,with the mask containing apertures therein. If properly constructed andpositioned, the aperture mask greatly reduces brightness and glare,reduces irradiation, and eradicates diffraction effects, therebyimproving contrast and avoiding undesirable bleeding of bright areasinto dark areas. Generally, an aperture mask works by reducing theresolution of a large scope down to the resolution of an unobstructedrefractor having the same size as the aperture in the aperture mask.Often, however, it can be desirable to have a sharp, steady image withreduced resolution instead of a bright image lacking any details. Theuse of an aperture mask provides this desired sharp, steady image with alarge scope.

Conventionally, aperture masks are deposited in a similar fashion tooptical coatings themselves. A first layer is deposited onto an opticalcoating. A chemical compound is then applied to the first layer in adesign corresponding to the desired aperture. This forms a mask on thefirst layer, masking away those portions that are to remain. An etchingcompound is then applied, which removes the non-masked portions of thefirst layer. The chemical compound is then removed via application of asuitable chemical, leaving the aperture mask formed from the firstlayer. This process requires the use of optical Dark Mirror Coating(DMC) coatings upon which the aperture mask is printed, and furtherrequires certain conditions, such as temperature and stress as well as along lift-off process for the chemicals.

It would therefore be desirable to provide aperture masks that eliminatethe need for optical DMC coatings and eliminate certain conditionsrequired for coatings, such as temperature and stress as well as thelong lift-off process in chemicals. Some improved aperture masks aredisclosed herein.

BRIEF DESCRIPTION

The present disclosure relates to methods and apparatuses (e.g., amulti-spectral optical filter array, an optical wafer, an opticalcomponent) having aperture masks deposited/printed thereon.

Disclosed in various embodiments are apparatuses comprising a substratehaving an aperture mask printed on at least one of a light entrancesurface or a light exit surface of the substrate so as to provide anaperture over a portion of the substrate. The aperture mask includes aphotoresist, or in other words is formed from the photoresist.

The photoresist may be opaque. In particular embodiments, the substratedoes not include an optical coating between the aperture mask and the atleast one of light entrance surface or the light exit surface to whichthe aperture mask is printed. The aperture mask can be printed on boththe light entrance surface and the light exit surface of the substrate.

The film-coated substrate can be a multi-spectral optical filter array,an optical wafer, or another optical component.

The photoresist can be positive or negative, and can be photo-definableor non-photo-definable.

Methods are also disclosed, the methods comprising providing afilm-coated substrate, printing an aperture mask on an optical coatingon at least one of a top or a bottom of the film-coated substrate so asto provide an aperture over a portion of the substrate, and depositing aphotoresist, the photoresist printed to form the aperture mask on thesubstrate.

The method can further comprise curing (e.g., via an ultraviolet lamp)the deposited photoresist on the substrate, thereby forming the aperturemask thereon.

Also disclosed herein are multi-spectral optical filter arrays having afilter element substrate with an opaque aperture mask printed thereon,the aperture mask being formed from a photoresist.

These and other non-limiting characteristics of the disclosure are moreparticularly disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 diagrammatically show a side view of a first exemplary filterarray having an aperture mask that includes or is formed from aphotoresist, which can be printed thereon.

FIG. 2 diagrammatically show a perspective view of the filter array ofFIG. 1.

FIG. 3 diagrammatically show a method of manufacturing the filter array.

DETAILED DESCRIPTION

A more complete understanding of the components, processes andapparatuses disclosed herein can be obtained by reference to theaccompanying drawings. These figures are merely schematicrepresentations based on convenience and the ease of demonstrating thepresent disclosure, and are, therefore, not intended to indicaterelative size and dimensions of the devices or components thereof and/orto define or limit the scope of the exemplary embodiments.

Although specific terms are used in the following description for thesake of clarity, these terms are intended to refer only to theparticular structure of the embodiments selected for illustration in thedrawings, and are not intended to define or limit the scope of thedisclosure. In the drawings and the following description below, it isto be understood that like numeric designations refer to components oflike function.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art. In case of conflict, the present document, includingdefinitions, will control. Preferred methods and materials are describedbelow, although methods and materials similar or equivalent can be usedin practice or testing of the present disclosure. All publications,patent applications, patents, and other references mentioned herein areincorporated by reference in their entirety. The materials, methods, andarticles disclosed herein are illustrative only and not intended to belimiting.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used in the specification and in the claims, the term “comprising”may include the embodiments “consisting of” and “consisting essentiallyof.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,”“contain(s),” and variants thereof, as used herein, are intended to beopen-ended transitional phrases, terms, or words that require thepresence of the named ingredients/steps and permit the presence of otheringredients/steps. However, such description should be construed as alsodescribing compositions or processes as “consisting of” and “consistingessentially of” the enumerated ingredients/steps, which allows thepresence of only the named ingredients/steps, along with any unavoidableimpurities that might result therefrom, and excludes otheringredients/steps.

Numerical values in the specification and claims of this applicationshould be understood to include numerical values which are the same whenreduced to the same number of significant figures and numerical valueswhich differ from the stated value by less than the experimental errorof conventional measurement technique of the type described in thepresent application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of “from 2 grams to 10grams” is inclusive of the endpoints, 2 grams and 10 grams, and all theintermediate values).

The terms “about” and “approximately” can be used to include anynumerical value that can vary without changing the basic function ofthat value. When used with a range, “about” and “approximately” alsodisclose the range defined by the absolute values of the two endpoints,e.g. “about 2 to about 4” also discloses the range “from 2 to 4.”Generally, the terms “about” and “approximately” may refer to plus orminus 10% of the indicated number.

The present disclosure may refer to temperatures for certain processsteps. It is noted that these generally refer to the temperature atwhich the heat source (i.e., furnace, oven, etc.) is set, and do notnecessarily refer to the temperature that must be attained by thematerial being exposed to the heat.

The term “room temperature” as used herein refers to a temperature inthe range of 20° C. to 25° C.

It is noted that the coefficient of thermal expansion is typicallyreported as the average between a starting temperature and a reportedtemperature.

It is further noted that as used herein, “aperture mask,” “mask,” and“opaque coating” may be used interchangeably, unless understood by thecontext in which they are used below to refer to distinct embodiments.For example, an “aperture mask” may comprise other coatings besides“opaque”, and the use herein is intended solely to assist the reader andnot to limit application of the subject disclosure to only aperturemasks of opaque coating materials.

As explained in greater detail below, the present disclosure providesexemplary embodiments of methods and apparatuses including theapplication of an opaque aperture mask that includes or is formed from aphotoresist on a variety of optical components (e.g., filter arrays).According to several embodiments set forth herein, the application maybe performed on one or both faces of the filter creating an aperturemask on the entrance and/or the exit face. It will be appreciated thatthe non-limiting examples of the present disclosure describe andillustrate embodiments wherein the aperture mask may be offset from theentrance face and/or the exit face, taking into account the incidentangle of a light ray.

As shown in FIG. 1, a preexisting or prefabricated optical filter array(i.e., the substrate) is provided. The substrate is, in particularillustrative embodiments, a multi-spectral optical filter array. Thesubstrate may comprise an optical wafer. The substrate of FIG. 1corresponds to an angled array. The perspective view of FIG. 2 shows the“stick” geometry of the optical filter elements of this one-dimensionalfilter array. As seen in both FIG. 1 and FIG. 2, the filter elementshave slanted sidewalls 140 (labeled only in FIG. 1). However, theoutermost optical filter elements of the filter array (those identifiedwith numbers “1” and “4”) have straight “outer” sidewalls 141 formingthe edges of the assembled filter array. This can be advantageousinsofar as the assembled filter array has the shape of a right-angledparallelepiped. An alternative (not shown) is to employ optical filterelements with both sidewalls slanted, and to include additionaltriangular-shaped fill elements to provide the assembled filter geometrywith straight outermost sidewalls. The filter array depicted in FIG. 1is shown as being an angled filter array. However, the substrates/filterarrays of the present disclosure can generally be of any desiredstructure and are only limited by the apertures masks which are appliedthereon.

With reference to FIG. 1, an improved filter array is shown inside-sectional view. In this diagrammatic illustrative example, a filterarray includes four filter elements labeled “1” to “4”. (This is merelyillustrative—in general the filter array may have dozens or hundreds offilter elements). Each filter element is diced from a filter plate onwhich a filter layers stack was deposited having a different opticalcharacteristic (e.g., different pass band or stop band, in terms ofcenter wavelength and/or bandwidth). As shown in FIG. 1, each filterelement can thus include a filter layers stack 112 supported by a filterelement substrate 114. The filter layers stack 112 is, for example,embodied as multiple layers of optical coatings forming interferencefilters, disposed on the filter element substrate 114.

Typically, each filter element is diced from a single filter plate. Thefilter elements may, in general, be designed for any pass band or stopband in the ultraviolet, visible, or infrared wavelength range. By wayof illustrative example, a filter element operating in the visible rangemay include a filter element substrate 114 of glass, sapphire, oranother material having suitable transparency in the optical range, andthe filter stack 112 may include alternating layers of tantalum oxide(Ta₂O₅) and silicon dioxide (SiO₂), or more generally alternating layersof two (or more) materials with different refractive index values. Byway of another illustrative example, the layers may be metal/metal oxidelayers such as titanium/titanium dioxide (Ti/TiO₂). Known techniques fordesigning interference filter optical stacks can be employed to designthe layer thicknesses for a given pass-band or notch filter stop-band,or to provide desired high pass or low pass filtering characteristics.The diced filter elements are bonded together using an adhesive or otherbond 116. The bonded optical filter elements may comprise a plurality ofoptical filter elements defined by different interference filters. Theinterference filters of the optical filter elements may comprisepass-band filters or notch filters operating in (in various embodiments)the visible spectrum, the ultraviolet spectrum, and/or the infraredspectrum.

With continuing reference to FIG. 1, the illustrative filter array isdesigned to be illuminated by light. Again, the filter array depicted inFIG. 1 is shown as being an angled filter array, such that the lightwould enter the array at an angle. As explained above, as will beappreciated by those skilled in the art, the substrates/filter arrays ofthe present disclosure can generally be of any desired structure and areonly limited by the apertures masks which are applied thereon.

In use, light impinges on a light entrance surface 123 of the filterarray. Printed on the light entrance surface 123 of the filter array areentrance apertures 120. The entrance apertures 120 define an aperturemask on the light entrance surface 123 of the filter array. Inaccordance with the present disclosure, the aperture mask defined by theentrance apertures 120 includes a photoresist, with the aperturesapplied at predetermined locations on the array. The entrance apertures120 reduce optical cross-talk (e.g. block stray light) at the lightentrance surface 123 of the filter array.

The light then passes through the filter layers stack 112 of the filterelement and through the filter element substrate 114, and exits from alight exit surface 124 of the filter array. Printed on the light exitsurface 124 are exit apertures 122. The exit apertures 122 define anaperture mask on the light exit surface 124 of the filter array. Inaccordance with the present disclosure, the aperture mask defined by theexit apertures 122 includes a photoresist, with the apertures applied atpredetermined locations on the array. The exit apertures 122 reduceoptical cross-talk (e.g. block stray light) at the light exit surface124 of the filter array.

The light output from the light exit surface of each filter element isfiltered by the filter layers stack 112 of that filter element, and thusincludes only the spectral component of the incident light in thepass-band (or only the spectral component outside of the stop-band, inthe case of a notch filter; or only the spectral component above thecut-off wavelength in the case of a high-pass filter element; or onlythe spectral component below the cut-off wavelength in the case of alow-pass filter element; or so forth). In FIG. 1, the filter layersstack 112 of each filter element is disposed on the light entrancesurface of the filter element (or, more precisely, on the light entrancesurface of the filter element substrate 114). It is alternativelypossible to have the filter layers stack disposed on the light exitsurface, or to have filter layers stacks disposed on both the lightentrance and exit surfaces (either of the same type to provide sharperspectral bandwidth or cutoff, or of different types to provide morecomplex filter characteristics, e.g., two stop-bands in a two-band notchfilter).

As explained above, the entrance and exit apertures 120, 122, defineapertures masks on the light entrance surface 123 and light exit surface124 of the filter array, respectively, and are patterned opaque coatingsdeposited onto the boundaries between optical filter elements afterassembly of the filter elements. In particular, the aperture maskincludes a photoresist, or in other words is formed from thephotoresist. Due to the use of a photoresist, the aperture mask can beprinted directly on the light entrance or exit surfaces 123, 124 of thefilter array (i.e., without an optical coating applied between theapertures and the light entrance surface or light exit surface). Thisadvantageously eliminates the need for optical coatings between theaperture mask and the substrate, and further eliminates certainconditions required for coatings, such as temperature and stress as wellas the long lift-off process in chemicals. It will also be appreciatedthat such an implementation will assist in the elimination of straylight and crosstalk between optical bands.

Again, the aperture mask(s) includes or is formed from a photoresist,which advantageously obviates the need to apply optical coatings to thesubstrate. Depending on the desired application of the filter array, thephotoresist can be negative or positive. Similarly, again depending onthe desired application of the filter array, the photoresist can bephoto-definable or non-photo-definable (e.g., a polyimide that is notlight sensitive, i.e. is not a positive photoresist and is not anegative photoresist). Suitable examples of positive photoresistscapable of functioning as set forth above include, for exemplarypurposes only and not for purposes of limiting the same: SK-9010,S-1813, or S-1818. Suitable examples of negative photoresists capable offunctioning as set forth above include, for exemplary purposes only andnot for purposes of limiting the same: AZ P4620, AZ NLof 2020, or AZNLof 2070. Suitable examples of non-photo-definable photoresists capableof functioning as set forth above include, for exemplary purposes onlyand not for purposes of limiting the same: polyimides.

As will be appreciated by those skilled in the art, application of anaperture mask including a photoresist to a substrate in accordance withthe present disclosure can be achieved by any suitable means. Forpurposes of example and not for purposes of limiting the same, theaperture mask can be applied in any desired pattern using additivemanufacturing techniques, such as printing via an inkjet-type additivemanufacturing printer, printing via an extrusion-type printer (i.e., afused filament fabrication printer), printing via any other 3-D printingtechnique, fused deposition modeling, or any standard photolithographytechnique, including but not limited to contact printing, sprayapplications, or any other exposure or development method. A separatemask is then used to expose the photoresist to light. The photoresistcan then be developed by application of a developer, which removes theundesired portion of the photoresist layer, leaving the desired portionbehind as an aperture mask. In other words, the photoresist is used toform the aperture mask, rather than used as a means of forming thedesired aperture mask pattern in an optical DMC coating and then beingremoved from the optical DMC coating.

Thus, in some embodiments an optical device comprises a multi-spectraloptical filter array comprising a plurality of optical filter elementsbonded together to form the multi-spectral optical filter array, and anaperture mask formed on a light entrance surface and/or on a light exitsurface of the multi-spectral optical filter array wherein the aperturemask comprises a photoresist or a non-photo-definable polyimide. In someembodiments, each optical filter element comprises a filter elementsubstrate and a filter layers stack forming an interference filterhaving a pass band or stop band, the filter layers stack supported bythe filter element substrate. In some embodiments each optical filterelement has a different pass band or stop band.

With continuing reference to FIGS. 1 and 2 and with further reference toFIG. 3, an illustrative method of manufacturing the aperture mask of afilter array is described. In an operation S1, a photoresist layer isdeposited on a surface of the optical substrate, that is, on thepreexisting or prefabricated optical filter array. For fabricating theentrance apertures 120, the photoresist layer is suitably deposited onthe light entrance surface 123 of the filter array. For fabricating theexit apertures 124, the photoresist layer is suitably deposited on thelight exit surface 124 of the filter array. In an operation S2, portionsof the photoresist layer are exposed to light. In an operation S3, thephotoresist layer is developed to form the aperture mask (e.g. entranceapertures 120 and/or exit apertures 122) on the surface of the opticalsubstrate.

The apparatuses (e.g., a multi-spectral optical filter array, an opticalwafer, an optical component) of the present disclosure can bemanufactured by any suitable means, as will be appreciated by thoseskilled in the art. For example, manufacture of the filter array of FIG.1 and FIG. 2 can include fabrication of a filter plate on a bulksubstrate for each filter element 1-4. Typically, this entails disposingthe substrate (e.g. a glass substrate for some visible-range designs) ina deposition system and depositing the filter layers stack bysputtering, vacuum evaporation, plasma deposition, or another technique,with the thicknesses of the constituent layers of the filter stack ofeach filter plate designed to provide filter characteristics of thecorresponding filter type. The result of this processing is a set offilter plates, e.g. four filter plates corresponding to filter elements1, 2, 3, and 4, for fabricating the illustrative filter array of FIG. 1and FIG. 2. Filter elements of the desired types can then be mounted ina bonding jig and glued together at the sidewalls (e.g., slantedsidewalls for an angled array, such as that depicted in FIG. 1 and FIG.2) using adhesive or are otherwise bonded together to form themultispectral filter array. Finally, other components, such as theentrance and/or exit apertures 120, 122 and other components (e.g.,light detectors) are added to the filter array to form a completemultispectral optical system.

The present specification has been set forth with reference to exemplaryembodiments. Modifications and alterations will occur to others uponreading and understanding the present specification. It is intended thatthe present disclosure be construed as including all such modificationsand alterations insofar as they come within the scope of the appendedclaims or the equivalents thereof.

The invention claimed is:
 1. An optical apparatus, comprising: asubstrate having a light entrance surface and a light exit surface;wherein the substrate is one of: a multi-spectral optical filter arraycomprising a plurality of optical filter elements bonded together toform the multi-spectral optical filter array, wherein each opticalfilter element includes a filter layer stack, or an optical wafer; andan aperture mask that includes a photoresist printed directly on thelight exit surface of the substrate so as to provide an exit apertureover a portion of the substrate; wherein no optical coating is presentbetween the substrate and the aperture mask.
 2. The apparatus of claim1, wherein the photoresist is opaque.
 3. The apparatus of claim 1,further comprising a second aperture mask that includes a photoresistprinted directly on the light entrance surface of the substrate so as toprovide an entrance aperture over a portion of the substrate.
 4. Theapparatus of claim 1, wherein the photoresist is photo-definable.
 5. Theapparatus of claim 1, wherein the photoresist is a positive photoresist.6. The apparatus of claim 1, wherein the photoresist is a negativephotoresist.
 7. An optical apparatus comprising: a multi-spectraloptical filter array having a filter layers stack supported by a filterelement substrate, the multi-spectral optical filter array including alight entrance surface and a light exit surface; an aperture maskprinted directly on a portion of the light entrance surface of themulti-spectral optical filter array, the aperture mask comprising aphotoresist; and a second aperture mask printed directly on a portion ofthe light exit surface of the multi-spectral optical filter array, thesecond aperture mask comprising a photoresist; wherein no opticalcoating is present between the multi-spectral optical filter array andthe aperture mask.
 8. The multi-spectral optical filter array of claim7, wherein at least one of the photoresists is opaque.